What is the performance impact of QA_ALLOW_UNINDEXED_QUERIES?

Question

I'm interested in the ability to show unanswered queries at a category level. According to the config documentation, enabling this flag could be costly. Does anyone have any experience in how expensive these queries are?

 

Set QA_ALLOW_UNINDEXED_QUERIES to true if you don't mind running some database queries which are not indexed efficiently. For example, this will enable browsing unanswered questions per category. If your database becomes large, these queries could become costly.

Comments

I also want to hear owner impression of large-scale sites that this option is true.